Prosocial behavior is a core aspect of individuals' well-being and healthy psychosocial and physical development. The purpose of the proposed project is to advance our limited understanding of the early development of this important behavior by specifying patterns of growth and testing specific mechanisms contributing to its development in the second and third years of life when prosocial responses first emerge. Better understanding of early prosocial responsiveness holds promise for identifying early markers for developmental disorders like autism which would permit earlier intervention efforts, and for promoting long-term positive adjustment in very young children who exhibit heightened aggression and externalizing behavior. The proposed research has three aims: 1) to map the developmental course of prosocial sharing between 12 and 30 months of age in relation to adults' communicative support and the sharing recipient; 2) to determine how two potential underlying mechanisms relate to the growth of prosocial sharing: developing social understanding, and mothers' talk with their children about others' mental states; 3) to determine whether young children's developing social understanding mediates the effects of mothers' talk about mental states on children's developing propensity to share. Using complementary cross-sectional and longitudinal designs, a standard set of sharing tasks will be administered to 40 - 120 children at each of 7 ages (12, 15, 18, 21, 24, 27, and 30 months), with either mother or a female experimenter as potential recipient. All tasks are similar in that the child has an overabundance of a material resource (e.g., cars, stacking rings) while the play partner and potential sharing recipient has none. The partner will provide a graded series of cues as to her need and desire, which will culminate in an overt request. Children will be scored for whether they share or not and if so, at what point in the series of cues. They will also be administered a series of tasks to index three conceptually related aspects of early social understanding: self-other understanding, emotion understanding, and ownership understanding. Finally, in the longitudinal sample, parents will read wordless picture books to their children which depict various emotions along with other content; parents' emotion and mental state talk will be transcribed from these. Analyses will identify age differences in the primary constructs (sharing; social understanding; parents' mental state talk). They will also identify concurrent and predictive associations between sharing and social understanding, and between sharing and parent mental state talk, above and beyond age and control variables such as family demographic characteristics, gender, temperament, parent-child relationship quality, and child compliance. Finally, they will address whether differences in social understanding mediate the effects of mothers' emotion and mental state talk on developing prosocial behavior. Based on a unique integration of the small but intriguing research on early developments in prosocial behavior, social understanding, and parents' mental state discourse, and using novel tasks and procedures, the proposed project will constitute the first systematic, controlled, experimental study of the emergence and early development of sharing and its relation to developing social understanding; the first examination of socialization correlates of sharing in this age period; and the first to determine whether and how social understanding in very young children mediates socialization effects on prosocial behavior. PUBLIC HEALTH RELEVANCE: Prosocial behavior is a core aspect of individuals' well-being and healthy development. Although much recent research has addressed the origins and early development of antisocial behavior, little is known about its complement, the origins of prosocial behavior. With greater understanding of early developments in prosocial behavior and specific socialization influences, we can begin to understand what may go awry in poorly understood developmental disorders like autism; inform preventive interventions for young children at risk for heightened aggression; and provide parents and educators with the requisite knowledge to help young children become more prosocial and to promote long-term health.